Install a 5 Blade Rotorhead (november 2010 added)
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The first step in learning about autorotation is to understand that rotor blades are rotary wings. We can all imagine an airplane flying without a motor (glider), but it's a bit more difficult to visualize a helicopter doing the same. By recognizing that each blade is a wing that acts like the wing of a glider, autorotation is more comprehensible. The blade element diagram can be used to understand the forces acting on the blade during an autorotation.
Blade Element Diagram
Normal Powered Flight
In May's edition of RC Heli, we defined the components of the blade element diagram and how they work during normal flight: The blade sees a combination of rotational flow (1) and downward induced flow (2) called relative wind (3). The angle of attack (4) is the angle formed between the relative wind and the chord line, and the pitch angle (5) is formed between the rotor plane and the chord line. Lift (6) is the total aerodynamic force perpendicular to the relative wind. For a helicopter in hovering flight, the lift vector is tilted aft. The lift vector can be broken into two components: a vertical component (7), which is the total force that generates vertical lift, and the rearward component called the induced drag (8), formed by the acceleration of air mass (downwash) and the energy spent in the creation of trailing vortices. Induced drag must be overcome to develop lift, and power is required to the rotor system to overcome this drag. The remaining vector on the blade element diagram is profile drag (9), a result of air friction acting on the blade element.
Blade Element Diagram
To keep the blades turning during normal flight induced drag must be overcome by engine power. During an autorotation, the power that turns the rotors comes from another source-potential energy (gravity), as the helicopter loses altitude. The rotor head will initially slow down, feeding on its own rotor inertia. Lowering collective will stop the decay. The increasing up flow of air through the rotor system reverses airflow. With
the lift vector always perpendicular to the relative wind, induced drag reverses and the lift vector is tilted forward providing a Pro-Autorotative force that turns the rotor head. A component of profile drag (In-Plane Drag) acts in opposite direction to the Pro-Autorotative force.
The Autorotative Regions of the Blade
As the rotor blades travel around the arc, each part of the blade sees a different relative wind--from lowest velocity at the hub, to highest velocity at the rotor tip. This is because the rotor tip has to travel farther in the same period of time as the part of the blade near the hub. During an autorotation the rotor blade sees three different regions: prop, auto, and stall region.
Auto Region Rotational speed combines with induced flow, shifting the relative wind below the rotor plane. Notice that the lift vector is tilted forward, providing a pro-autorotative force. This region is increased and shifted toward the blade tip at higher pitch settings, decreasing rotor rpm and slowing the sink rate.
Prop Region Relatively high rotational speed at the outer portion of the rotor disk combines with induced flow, shifting the relative wind towards the horizontal. Notice the lift vector is tilted more vertical than forward, providing less pro-autorotative driving force. In this region, the profile drag is the largest and causes greater anti-autorotative force. This region is increased with lower pitch settings and higher rpms, thus reducing the auto region, resulting in a faster sink rate.
Stall Region The stall region is at the blade root, where rotational flow is reduced to the point where lift is not generated and profile drag dominates. As pitch angle is increased, rotor rpms are reduced and the stall region increases across the blade, reducing the auto region and prop region.
As blade pitch and rpm changes, the three regions change across the blade. It is very important that during an autorotation, pitch angle and rpm are controlled for the most practical use of the blade regions.
When the motor decides to quit at the most inconvenient time, the lift vector is pointed aft, quickly slowing the rotor head without the motor to overcome the induced drag. The pilot quickly lowers the collective to decrease the stall region on the blade.
During the descent, the pilot starts to control the pitch of the blades in order to adjust the amount of prop and auto region. If the pilot wants more lift, he adds collective, slowing the rotor head and increasing lift. For a higher sink rate, the pilot decreases pitch, increasing rotor rpm. This balancing act is optimized in order to find a suitable rate of descent that will get the pilot to the desired landing point.
Getting close to the ground, the pilot trades airspeed for rotor head power by flaring, increasing the reversed induced flow through the rotor head, increasing the lift vector, and tilting it more forward, causing a higher rotor rpm.
As airspeed is traded, the helicopter starts to settle. At this point the pilot trades energy from rotor inertia into greater lift. Hopefully the pilot didn't go too deeply into the bank account of rotor energy. Before the blades stall, the helicopter should settle safely on the ground.
I hope this took some of the mystery out of autorotation. By adjusting pitch you control the direction and magnitude of the relative wind, lift vector, and pro-autorotative force. Proper collective management through the decent, flare, and touch down will save you from buying a new set of skids and will result in a magnificent crowd-pleasing auto.
How to use Dampers to best effect
Head dampening is one of those very important aspects of a helicopter that often gets downplayed or overlooked. Knowledge of dampening basics and its purpose in a model heli will enable any pilot to better modify, fly, and understand the machine.
What is Damping?
Head damping, or dampening as it is sometimes called, serves a number of purposes in a model helicopter. To better understand damping, it is good to look to the full size helicopter's equivalent, flapping. Head flapping is basically defined as the act of a rotor blade moving to change its angle of attack in flight. The reason the blade moves is to balance the system. When one blade is moving into the wind and the other is retreating back to the tail in forward flight, an imbalance occurs. The retreating blade has less lift because the air speed hitting the blades is less than the airspeed hitting the advancing blade. With a flapping head the retreating blade will raise up to increase its angle of attack on the wind while the advancing blade will flap down to lower its lift angle. This happens until there is balance between the lift and the centrifugal force acting on the blades. Without this balancing act, the heli would be very unstable in flight and could shake itself apart! Flapping is achieved in a model heli in two ways: The flex of the blades/blade grips, and via dampers/hinges.
Since the majority of modern model helis--as many as 90 percent--use a spindle with dampers connecting the blade grips together, the discussion will be limited to that style of setup, which is called a floating head setup. There are true flapping setups used in model helis, but they are limited in number. This is primarily due to the parts count, complexity, and
increased cost over the floating head design. A number of high end FAI machines use flapping heads with great results and very stable flight characteristics, but for most helis and pilots the gains are not worth the costs.
Stiff or Soft?
What can be accomplished with this basic understanding of damping? The answer is, a lot! Besides keeping the helicopter from unstable flight or destroying itself, dampers can also be used to adjust the flight characteristics of a heli. Which brings up the standard question regarding heli dampers: "How hard should they be?" Dampers for the same heli often come in a variety of stiffness ratings (usually measured as the durometer rating). The higher the durometer rating, the firmer the O-ring or damper will be.
Why use Firm Dampers?
A firm damper forces the helicopter to react faster to cyclic inputs, a.k.a. the fuselage will follow the rotor head faster, while a softer damper will react a little slower to the input. Please note that the firmness of the damper only affects the initial movement. For example, if two pilots started doing continuous flips with equal machines and setups with different dampers (remember, this is hypothetical as no two machines and pilots are alike) the one with the harder dampers would react faster, but both machines would be flipping at the same speed after the initial input. To keep it simple, firmer dampers will give quicker response to an input, but do not affect the total end response. Firm dampers also offer better protection against boom strikes with the helicopter as they don't allow the main rotor disk to deflect as much. One tip that can help firm up a damper system is to install shims between the blade grips and the rotor head on the spindle. When the spindle bolts are tightened, these shims will tighten against the dampers and compress them, making a tighter setup. 3D pilots usually want the hardest damper they can get--within reason. Some pilots have even run a heli without dampers (just solid mounting of the spindle) with success, but this is not recommended as the heli can violently destroy itself if that damping isn't being done somewhere, especially considering how stiff the modern carbon main blades are.
Why use Soft Dampers?
After reading all the benefits of a firm damper above, one might think "Why would I consider softer dampers?" There are numerous reasons why a pilot would want softer damping. When dampers and blades are too stiff, the heli will wobble and nod in the hover and slow maneuvers. Also, a heli with stiff dampening is very twitchy and reacts to even the slightest transmitter stick movement, such as a pilot's jitters. Softer dampers are very stable in the hover and in nice slow moves, such as figure eights and scale style flying. Softer dampers will give the heli flight characteristics closer to a full size helicopter--not too many full sizes do 3D! Beginners tend to prefer softer dampers as the heli is smoother and slower to react, giving the pilot time to adjust and think. The main concerns with going too soft include possible boom strikes due to the retreating blade being forced down to the boom by a large cyclic input/low head speed, and more flutter in flight due to the blades flapping to a stall angle.
Here is a real life example of a heli and its damper choices. The Hirobo Freya Evo helicopter has two types of dampers available from the manufacturer, the stock ones are a 60 durometer and they also offer a set of 3D ones that are rated at 80 durometer. For a heavy 3D pilot, the 80 durometer would be the best choice, while an FAI precision type pilot or beginner may want the 60 durometer units.
There are a lot of damper options out there for most helis, including a number of aftermarket sets. Most manufacturers offer hard and soft version of their dampers. These will usually look alike but be of a different color. Some companies use O-rings, while others use soft plastic. The aftermarket companies offer an array of options, including:
Hard plastic dampers with O-rings on the outside. These dampers are about as stiff as it gets. The O-rings provide the stability needed for hovering and flight, but in a hard maneuver they will contract and the plastic bottoms out in the head block giving instant response. These types of dampers run very well in 3D flight. Their main drawback would be the fact that they can be damaged more easily when a crash occurs.
O-ring upgrades. These dampers are usually of a harder durometer rating than what the manufacturer offers and they are the whole damper. They generally consist of two or more O-rings that the spindle rides inside the head block. Available in many durometer ratings that are suitable for all flight types.
O-rings in a sleeve. Some manufacturers offer sets that have a number of smaller O-rings inside a metal sleeve that goes into the head block. These work much like the ones above.
With so many options out there for dampers on most machines, there will be a set that will make your heli fly the way you want it to. As to which to choose, look at your flight style and what other pilots with the same heli are using, and that should be a great place to start.
How to Change and Install Dampers
Dampers are generally installed at each end of the head block with the spindle riding inside them. There are some helis that have the dampers in the head block at a teetering point, but the general installation and setup is the same. Most dampers should be lubricated with a type of grease that won't eat the rubber. I use dielectric tune up grease (found at an auto parts store) with great results.
1) Coat the inside and outside of the damper with a thin layer of the grease and install into the head block.
2) Next, lube the spindle shaft with the same grease and slide it into the dampers. I like to put a piece of paper towel or cotton swab into the spindle at the end to keep any grease from getting in the bolt threads--I'd hate to have a spindle bolt free itself in flight--very ugly. The grease allows the spindle to move in the head block so it can center in flight.
3) Install the blade grips and it is ready to go. There are some dampers made with a special plastic that should not be lubricated, and as always, see the instructions first.
Dampers need to be checked often for wear and damage. The main rotor takes some abuse, and the dampers carry the whole assembly! To check the dampers without removing anything, simply grab the blade grips and rock them in the head block. If there is significantly more movement than when the dampers were new, it may be time to replace them. The heli can also exhibit tracking and fluttering problems when dampers are worn out, so watch for those signs. As always, check them after a crash too.
Dampers play a very key role in the RC helicopter world. Good dampening systems contribute greatly to the stability and predictability of the modern heli. Adjusting the dampers on the helicopter can have a great affect on how it flies and gives the pilot another place to fine tune a machine to his/her liking. Enjoy!